1. Field of the Invention
The present invention relates to a polishing disc suitable for polishing an optical fiber end surface into a spherical surface and method for polishing a spherical surface using such polishing disc.
2. Prior Art
Various proposals have been made with respect to polishing devices and polishing discs for polishing an end surface of an optical fiber into a spherical surface.
Spherical polishing of an optical fiber end surface may be achieved by using an apparatus according to patent applications (Japanese Patent Laid-Open No. 62-173159/1987) "method of processing an end surface of rod and apparatus therefor" by Nippon Telegraph and Telephone, and (Japanese Patent Laid-Open 3-81708/1991) "polishing method of ultra low reflection optical connector ferrule" by the same.
These polishing devices will be briefly described below with reference to FIG. 5 and FIG. 5A.
An optical fiber 2 of which the tip end is to be polished into a spherical surface is inserted into the center hole of a ferrule 1 and is adhered thereto. The ferrule 1 is then supported by a ferrule holder 9 and its tip end is pressed against a polishing plate which will be described later.
The ferrule 1 is turned through a turning angle of 180.degree. both left and right in a reciprocating manner as indicated by arrows by means of a driver mechanism (not shown).
As shown, a tip end surface 3 of the ferrule 1 is formed in a pre-processing procedure into the shape of a cone. A hollow rotating drum 4 which rotates at a high speed is formed integrally with a rotating shaft 5 at its center. A hard plastic film disc 6 is held at its peripheral portion 7 by a holding ring member 8 so as to be mounted on the hollow rotating drum 4.
Since a ferrule tip end 10 is pressed against the hard plastic film disc 6 by a polishing load (P), the portion of a contacting point Q between the plastic film disc 6 and the ferrule tip end 10 is locally deformed to have a section exhibiting a circular arc.
In this state, by effecting the reciprocating turning of the ferrule 1 while dropping abrasive 11 on the upper surface of the plastic film disc 6, a fine portion at the apex of the cone tip of the ferrule 1 is polished into a spherical surface.
Furthermore, the present inventor has proposed "OPTICAL FIBER END-SURFACE POLISHING DEVICE" (Japanese Patent Laid-Open No. 3-26456/1991: U.S. Pat. No. 4,979,334). An optical fiber end surface may be polished into a spherical surface by using the polishing device. This apparatus will be described below with reference to FIG. 6.
The tip end of a ferrule 13 having a center hole into which an optical fiber 12 is inserted and fixed by means of adhesion is pressed against a polishing disc and is polished by a relative movement for polishing.
A turn table 14 revolving at a high speed describes a circular locus having a rotating radius R around the center of revolution at a center axis 15. The turntable is rotated by means of a driver mechanism (not shown), and at the same time is caused to rotate on its own axis at a very low speed. A polishing plate 16 of an elastic material is placed on an upper surface of the turn table 14, and a polishing film 17 having a soft plastic film surface with abrasive applied thereon is pasted onto the upper surface of the polishing plate 16.
While being pressed against the surface of the polishing film 17 by a polishing load (W), a tip end surface 18 of the ferrule 13 is urged downward and held still by a ferrule holder 19, and, in this state, polishing is effected by causing the turn table 14 to both revolve and rotate.
Accordingly, the tip end surface 18 of the ferrule 13 is concentrically polished and first removed, as the polishing load acts is the outer periphery of the end surface of the ferrule 13, due to the fact that the surface of the polishing film 17 is pressurized to cause a flexible deformation by the polishing load (W). The polishing and removing process gradually proceeds toward the center of the ferrule 13. When a uniform polishing pressure acting upon the end surface of the ferrule 13 has been achieved, the end surface of the ferrule 13 is formed into a spherical surface and the spherical polishing is completed.
In general, polishing and removing ability of the plastic film disc used in the polishing device as described with reference to FIGS. 5 and 5A is extremely low due to its structure.
In the above described apparatus of the conventional example, therefore, in order to supplement the polishing and removing ability of the plastic film disc 6, the tip end 10 of the ferrule 1 is previously formed into the shape of a cone and the portion to be polished and removed by the plastic film disc 6 is limited to a fine portion at the tip end of the cone. On the other hand, it is known that the amount of light reflection occurring at the optical fiber end surface is increased in proportion to the polished surface roughness of that surface. It is also known that, in addition to the grain size and material of abrasive grains, the polishing pressure largely affects a reduction in the roughness of the polished surface.
However, in this conventional example where the polishing area of the tip end portion of the ferrule 1 is a very small pinpoint-like area having a diameter on the order of 100 .mu.m, a fine pressure control for properly keeping the required polishing pressure is next to impossible. If the polishing pressure is not suitable, small scratches occur on the polished surface and it is thus difficult to obtain an excellent polished surface.
It may be said that optical loss due to reflection in a returning direction obtained by this conventional polishing method is on the order of 40 dB, and an optical loss due to reflection in a returning direction of 55 dB cannot be achieved, which is thought to be required in a large capacity optical fiber communication in the future.
In the conventional apparatus according to the proposal by the present inventor as described with reference to FIG. 6, the tip end diameter of the ferrule 13 is generally regulated to 1.2.about.1.9 mm to provide a sufficient tip end area for the fine adjustment of the polishing pressure (W), and the polishing film 17 is retained by a thick polishing plate 16.
Accordingly, since a relatively large polishing force (W) may be used, fine adjustment of the pressure is easier compared to the foregoing example.
When, for purpose of comparison, a polishing film having equivalent abrasive grains as that in the example of FIGS. 5 and 5A is used, optical loss due to reflection return occurring at the end surface of an optical fiber was generally 48 dB, showing a great improvement. It was difficult, however, to stably obtain 50 dB or above.
One of the reasons why optical loss due to reflection return cannot be reduced as described is presumably because of deterioration in the polishing boundary conditions, which occurs as the polishing process proceeds because of the structure of the polishing film.
FIG. 4 shows an enlarged sectional view of a polishing film having a base made of an ordinary plastic film. This polishing film is manufactured such that an abrasive powder 21 is mixed with a resinous adhesive binder agent 22, is applied uniformly in a thin layer, and then dried on one surface of a plastic film 20. When behavior of the polishing film at the time of polishing of the ferrule is observed, the lower surface of the ferrule and the polishing film surface are slid relative to each other in the state where a polishing pressure is continuously added. As the lower surface of the ferrule is gradually polished and removed, the applied layer of the abrasive grains 21 on the polishing film, too, is gradually removed at the same time.
It was thus found that, as the polishing process proceeds, powders removed from the ferrule, free abrasive grains or resinous binder agent, etc., which have been scrubbed off and pulverized, are joined together to form an accumulation of fine masses and at the same time are irregularly buried in the adhesive binder agent on the polishing film surface. Thus, the surface roughness of the polishing film becomes nonuniform and is rapidly degraded. Further, the abrasives adhere to the periphery of a chip scrubbed from the resinous adhesive binder agent to form a free abrasive grain having a large apparent diameter. This is harmful.
Therefore, with the conventional method using a polishing film having abrasive grains applied thereon, the ferrule end surface is polished by a polishing film surface which is degraded as the polishing process proceeds and, as a result, there is a limit in the smoothness of the polished surface.
As described, spheric polishing at the tip end of an optical fiber by a typical conventional optical fiber end surface polishing device is with limitation, and it has been impossible to stably achieve a reflection return optical loss of 50 dB or above.
Returning light by reflection occurring due to the roughness in the polished surface of the connecting end surface of the optical fiber must be minimized, since the operation of a laser source becomes unstable when it is fed back to the laser source. In the case of a high speed, large capacity optical communication system, a reflection return optical loss of 50 dB or above is required. In order to satisfy this requirement, spheric polishing of the optical fiber end surface must be done to such an extent that the reflection return optical loss is 50 dB or above.